The present invention relates generally to the field of paper making and, in particular, to the manufacture of release base papers. More specifically, the invention relates to a process for incorporating nano-fibrillated cellulose fibers, also known as cellulose nanofibrils (CNF), into release base papers and the release papers made by this process.
Release base papers are the largest true specialty paper market, with a global market size of nearly 34 billion square meters, equating to approximately 2,700,000 tons of base materials. This includes both release and casting papers and filmic substrates. North America alone, uses over 750,000 tons of paper and 120,000 tons of film for release base in all applications.
“Release papers” are known in the art as a base paper having a silicone or other inert release agent coated on the surface of the base paper. In many applications, the release paper may serve as a substrate for a secondary layer. Examples of substrates with secondary layers include, for example, pressure-sensitive adhesive labels, and “casting substrates” for industrial polymeric or thermoplastic films. In other applications, the release paper may be used without a secondary layer, for example with certain food processes, such as baking cups and sheets or interlayers between sliced foods.
Release base papers require strength, a very smooth finish, low air permeability, and a high degree of coating holdout. Some applications also require that a release base paper have a high degree of translucency or transparency. Other applications require that a casting substrate remain dimensionally stable over a wide range of temperatures and humidities in order to withstand exposure to high temperature for curing of a silicone release coating of the materials cast on the sheet and to lie flat while the pressure sensitive material (usually a label or signage) is printed and applied to the object to be labeled or decorated.
Release base papers with low air permeability may be produced by using very low freeness pulps as part of the paper-making furnish. Low freeness pulps are heavily refined which retards paper machine productivity by slowing drainage during the sheet forming process, lowers dimensional stability of the final product, and increases manufacturing costs, including higher refiner energy and drying energy usage. Thus, generating the above mentioned properties in conventionally furnished papers requires high levels of energy usage, reduced machine operating speeds, and/or the use of petrochemical based content coatings, which includes extrusion coatings of polyethylene, or polypropylene, or 100% petrochemical based film—usually a polyester.
Plastic films or petrochemical based content coatings used in the prior art are directly affected by the price of oil, and as a result, their cost is subject to price fluctuation. Plastic films or petrochemical based content coatings are also not easily recycled, nor can they be disposed of with biodegradable materials; which further increases the disposal and total use costs.
Therefore there is a need in the art for a more energy and cost efficient process that provides for the manufacturing of release base papers and casting substrates, and materials to facilitate such a process.